The present invention relates to the field of administration of therapeutic and diagnostic agents in vivo, including methods and compositions for targeting such agents to particular types of cells.
The information provided herein and references cited are provided solely to assist the understanding of the reader and does not constitute an admission that such information or any of the cited references constitute prior art to the present invention.
The detection of a target site benefits from a high signal-to-background ratio of detection agent. Therapy benefits from as high an absolute accretion of therapeutic agent at the target site as possible, as well as a reasonably long duration of uptake and binding. The targeting ratio and amount of agent delivered to a target site can be improved using targeting vectors comprising diagnostic or therapeutic agents conjugated to a targeting moiety for preferential localization.
Examples of targeting vectors include diagnostic or therapeutic agent conjugates of targeting moieties, such as antibody or antibody fragments, cell- or tissue-specific peptides, and hormones and other receptor-binding molecules. For example, antibodies against different determinants associated with pathological and normal cells, as well as determinants associated with pathogenic microorganisms, have been used for the detection and treatment of a wide variety of pathological conditions or lesions. In these methods, the targeting antibody is directly conjugated to an appropriate detecting or therapeutic agent as described, for example, in Hansen U.S. Pat. Nos. 4,468,457, 4,444,744, 4,460,459, 4,460,561, 4,624,846 and 4,818,709, the disclosures of all of which are incorporated herein by reference.
One problem encountered in direct targeting methods, i.e., in methods in which the diagnostic or therapeutic agent (the “active agent”) is conjugated directly to the targeting moiety, is that a relatively small fraction of the conjugate actually binds to the target site, while the majority of conjugate remains in circulation and compromises in one way or another the function of the targeted conjugate. In the case of a diagnostic conjugate, for example, a radioimmunoscintigraphic or magnetic resonance imaging conjugate, non-targeted conjugate which remains in circulation can increase background and decrease resolution. In the case of a therapeutic conjugate having a toxic therapeutic agent, e.g., a radioisotope, drug, or toxin, attached to a long-circulating targeting moiety such as an antibody, circulating conjugate can result in unacceptable toxicity to the host, such as marrow toxicity or systemic side effects.
Pretargeting methods have been developed to increase the target:background ratios of the detection or therapeutic agents. Examples of pre-targeting and biotin/avidin approaches are described, for example, in Goodwin et al., U.S. Pat. No. 4,863,713; Goldenberg, U.S. Pat. No. 5,525,338; Goodwin et al., J. Nucl. Med. 29:226, 1988; Hnatowich et al., J. Nucl. Med. 28:1294, 1987; Oehr et al., J. Nucl. Med. 29:728, 1988; Klibanov et al., J. Nucl. Med. 29:1951, 1988; Sinitsyn et al., J. Nucl. Med. 30:66, 1989; Kalofonos et al., J. Nucl. Med. 31:1791, 1990; Schechter et al., Int. J. Cancer 48:167, 1991; Paganelli et al., Cancer Res. 51:5960, 1991; Paganelli et al., Nucl. Med. Commun. 12:211, 1991; Sharkey et al., Bioconjugate Chem 8:595-604, 1997; Stickney et al., Cancer Res. 51:6650, 1991; and Yuan et al., Cancer Res. 51:3119, 1991; all of which are incorporated by reference herein in their entireties.
In pretargeting methods, a primary targeting species (which is not bound to a diagnostic or therapeutic agent) is administered. The primary targeting species includes a targeting moiety which binds to the target site and a binding moiety which is available for binding to a binding site on a targetable construct. Once sufficient accretion of the primary targeting species is achieved, a targetable construct is administered. The targetable construct includes a binding site that recognizes the available binding site of the primary targeting species, and a diagnostic or therapeutic agent.
Pretargeting offers certain advantages over the use of direct targeting methods. For example, use of the pretargeting approach for the in vivo delivery of radionuclides to a target site for therapy, e.g., radioimmunotherapy, reduces the marrow toxicity caused by prolonged circulation of a radioimmunoconjugate. This is because the radioisotope is delivered as a rapidly clearing, low molecular weight chelate rather than directly conjugated to a primary targeting molecule, which is often a long-circulating species.
A characteristic encountered with some pretargeting methods is that circulating primary targeting species (primary targeting species which is not bound to the target site) interferes with the binding of the targetable conjugate to targeting species that are bound to the target site (via the binding moiety on the primary targeting species). In some methods, the level of circulating primary targeting species is reduced, such by using a clearing agent that binds to the primary targeting species and facilitates clearing that species from circulation. An example is described in Goodwin, et al., U.S. Pat. No. 4,863,713. However, reducing the level of circulating primary targeting species shifts the binding equilibrium, causing bound primary targeting species (which can be bound with targetable construct) to dissociate from the target, thereby reducing the period of time that the detection or therapeutic species is present at the targeted site.